Conventionally, as fire-retardant resin compositions, PVC or a polyolefin composition blended with a halogen-type fire-retardant, is mainly used. However, such compositions have problems such as, when the compositions are burned, large amounts of smoke and corrosive-gases, such as hydrogen chloride, are produced.
Therefore, recently, as fire-retardant resin compositions that evolve no corrosive-gases and evolve less smoke when burned, non-halogen-type fire-retardant resin compositions highly loaded with a metal hydrate, typically magnesium hydroxide or the like, are under study and are used practically in some fields.
In such compositions, because it is required to fill a base polymer with a high content of metal hydrate, as the base polymer, an ethylene/vinyl acetate copolymer or an ethylene/acrylate copolymer, with which the filler, such as magnesium hydroxide, can be easily blended, is mainly used. However, since the tensile strength of such compositions is on the order of about 10 MPa, and the melting point thereof is about 100.degree. C., the level of the properties is still low in comparison with the level of the properties (the tensile strength, about 15 to 20 MPa; and the ratio of heat deformation at 120.degree. C., about 10%) of the PVC currently mainly used to be extruded. Therefore, the development of non-halogen-type fire-retardant resin compositions having properties similar to those of the current PVC is desired.
On the other hand, electric wire used for automobiles, and electric wire used as internal wiring of electronic equipment, as standardized by JASO D611 and JIS C 3406, are required to have such various properties as fire retardancy, tensile properties, heat resistance, and abrasion resistance. Accordingly, as the covering material of these electric wires, PVC or polyolefin compounds blended with a halogen-type fire-retardant is mainly used.
However, since such electric wires have the problem stated in the above that, when they are burned, large amounts of smoke and corrosive-gases are produced, electric wires that evolve less smoke and no corrosive-gases when burned have recently been under study.
As a covering material of these electric wires, an ethylene-series copolymer (e.g., an ethylene/vinyl acetate copolymer and an ethylene/acrylate copolymer) loaded highly with a metal hydrate, has been studied. However the electric wire covered with that material is not satisfactory when used as electric wire for automobiles or as internal wiring of electronic equipment, in view of the heat resistance and mechanical properties, such as tensile strength and abrasion resistance.
Incidentally, although an example of an electric wire covered with a polypropylene composition excellent in mechanical properties and heat resistance is disclosed in Japanese Patent Application (OPI) No. 131052/1987, if an electric wire covered with this composition is used as electric wire for automobiles, as shown in the Example, it has the problem that the rupture stress is about 2 to 10 MPa, showing that the tensile strength is insufficient. Further, the rate of extrusion of the electric wire is only about 10 to 80 m/min, which is extremely low in practice in comparison with the rate of extrusion of an electric wire of 400 to 500 m/min, which is usually required industrially, and there is also the problem that the electric wire becomes expensive.
When the circumference of a conductor is covered with a layer of a composition highly filled with a metal hydrate, in the step of removing the covering layer at the end of the electric wire by a wire stripper or a casting machine, a residue of the covering layer remains, leading to the problem that a contact failure with the terminal onto which the electric wire is to be mounted occurs.